1. Field of the Invention
The present invention relates to a spatial light modulator, a spatial light modulator array, and an exposure apparatus using them.
2. Description of the Related Art
A spatial light modulator (SLM) is a device for forming an optical image corresponding to an electrical or optical input. One type of SLM is a digital micromirror device (DMD) in which a micromirror is fabricated by means of a micro-mechanics technique and which attempts to deflect light by tilting the micromirror. The DMD is a monolithic single chip integrated circuit SLM and is constituted of a high-density array of movable micromirrors, each of which measures, e.g., 16 microns square. These mirrors are formed on an address circuit, thereby forming a pixel of the DMD array. When set at one of two positions the mirrors reflect incident light toward a projection lens, and when set at the other position deflect the incident light toward a light absorber. The projection lens forms an image by means of focusing modulated light on a display screen.
The spatial light modulator described in JP-A-8-334709 has a rectangular mirror which is supported and raised upward on a yoke by means of a support post. The support post extends downward from the center of the mirror and is mounted to the center of the yoke along a torsion axis of the torsional post, thereby striking a balance of the mass of the mirror on the yoke. In this spatial light modulator, when a voltage is applied between the mirror and an overhead address electrode and between the yoke and a lower address electrode, the mirror is rotated by means of electrostatic force developing between the mirror and the overhead address electrode, and the yoke is rotated by means of electrostatic force developing between the yoke and the lower address electrode. One of landing pieces of the yoke lands on the lower address electrode and is supplied with the voltage, thereby enabling free side-to-side rotation.
However, since the landing piece of the yoke comes into contact with the lower address electrode, there arises van der Waals force or adhesion of the landing piece to the address electrode by means of impurities or an organic-based gas in the environment, thereby resulting in a problem of a decrease in the reliability of an element.
In order to solve such a problem, as shown in FIG. 37, a spatial light modulator 1 described in JP-A-2001-242395 comprises a mirror 7 attached to a yoke 5 by means of a mirror support post 3; mirror address electrodes 9; and yoke address electrodes 11. Trap electrodes 13 are newly added to the same locations where the mirror address electrodes 9 are formed. During the course of operation, the mirror 7 is rotated by means of attractive force stemming from electrostatic force developing between the mirror 7 and the mirror address electrode 9 and between the yoke 5 and the yoke address electrode 11. The mirror comes to a stop at a desired angle by means of development of strong attractive electrostatic force between the edge of the yoke 5 and the added trap electrode 13.
FIG. 38 is a perspective view of an end-coupled DMD structure in which the mirror 7 is drawn in perspective. This drawing also shows a torsion hinge 15, which shows a situation where the attractive force acting on the (left-side) lower edge of the rotating mirror 7 is greater than the attractive force acting on the (right-side) upper edge of the rotating mirror 17. Here, the yoke 5 and the left-side trap electrode 13 in the drawing are in extreme proximity to each other. The attractive force corresponds to a function of electrostatic capacity and distance, both existing between the edge of the yoke 5 and the edge of the trap electrode 13. This function is highly dependent on the area on the end face of the yoke 5, the area of the end face of the trap electrode 13, and a distance between the respective end faces. This distance is expressed as f=1/d2, in the manner of a second-order function. By means of this configuration, the mirror 7 rotates along an orthogonal axis situated in the overall edge of the mirror 7 or the yoke 5 which is adjacent to and located very close to the trap electrode 13.
Accordingly, in this DMD structure, the trap electrode 13 is provided so as to come into proximity to the continuous edge of the mirror 7 or the yoke 5 when the mirror 7 is rotated to an appropriate height above the surface of the element and a domain of a desired angle. Hence, extremely strong electrostatic attractive force develops between the trap electrode 13 and the mirror assembly. When the mirror 7 has come close to the plane of the trap electrode 13, the mirror 7 is biased so as to come to a stop. As a result, the mirror 7 is attenuated without fail by means of adjusting the pulse waveform of the mirror assembly, whereby the mirror assembly is stopped after having moved through a desired rotational angle.
In the light deflector described in Dual-Mode Micromirrors for Optically-Phased Array Applications TRANSDUCERS 01 EUROSENSORS XV [The 11th International Conference on Solid-Sensors and Actuators (2001)], a mirror is fixed to the center of a pivotal shaft, and orthogonal support shafts are fixed to both ends of the pivotal axis. A so-called comb drive, which is drive means, is provided on the respective ends of the support shafts. In the comb drive, a comb-shaped upper electrode plate and a comb-shaped lower electrode are oriented while being fitted together. Both ends of the respective support shafts are vertically actuated by means of electrostatic force acting on the mutually-opposing electrodes. Consequently, the pivotal shaft is rotated, to thus enable free side-to-side rotation of the mirror.
However, the spatial light modulator described in JP-A-2001-242395 has a yoke and a mirror, which are provided on a torsion hinge in a two-stage structure. As a result, the overall mass of the movable section is increased, thereby posing a limitation on high-speed movement. The address electrodes and the yoke are disposed in the same plane, so that the areas of the respective electrodes become smaller. As a result, the electrostatic attractive force acting on the pixel mirror and the address electrodes and the electrostatic attractive force acting on the yoke and the address electrodes become smaller, thus posing a limitation on the tendency toward lower voltage and higher-speed driving.
Further, since a single trap electrode operates only the edge, there arises an inconvenience of a reduction in a range of action of an electric field to be trapped.
Moreover, in addition to the complicated structure of the comb drive, the light deflector described in Dual-Mode Micromirrors for Optically-Phased Array Applications TRANSDUCERS 01 EUROSENSORS XV [The 11th International Conference on Solid-Sensors and Actuators (2001)] requires four comb drives. Hence, the footprint of the comb drive structure becomes larger, thereby posing difficulty in attaining miniaturization and an increase in resolution and a problem of a low utilization factor of light.